Web processing rolls such as rolls used for handling and manipulating web of material and sheets formed from the web of material such as napkin folders, singlefold interfolders, and multifold interfolders all use vacuum to hold the web onto and transfer the web between rolls in the system. Additionally, some machines use vacuum to actually manipulate the web of material such as to make folds in the web of material.
All of these machines connect vacuum holes in the face of the rolls to a vacuum passage within the roll. The vacuum passage typically runs the length of the roll. Due to the width of some rolls, the vacuum passage is typically connected to a source of vacuum at both ends of the roll such that air flows in one direction (i.e. toward one of the ends) in one half of the vacuum passage and in the opposite direction (i.e. toward the other end) in the other half of the vacuum passage. However, in narrower embodiments, the vacuum source may be connected to a single end of the roll.
The source of vacuum will typically include valving for selectively turning on and off the vacuum supplied to the vacuum passage.
Pressure drop down the length of the axial vacuum passages is a significant problem as folders get wider and faster. The pressure drop manifests as reduced vacuum toward the center of the machine. The pressure drop is caused by axial vacuum passages too small for the air flow through them. Roll bodies do not have enough space to make the axial vacuum passages large enough to reduce the pressure drop.
Even when the cross-section of the vacuum passages is increased, such as in a tube-in-tube design, the pressure drop can be significant enough to effect vacuum performance.
The pressure drop down the length of an axial vacuum passage has at least two components. One component is friction between the flowing air and the passage wall. The other component is flow blockage caused by jets of air entering the vacuum passage from the holes in the roll face.
What is needed is a way to get more air flow with less pressure drop through the axial vacuum passages without making the vacuum passages larger.